It has been known for some time that injections of killed suspensions of anaerobic coryneforms (e.g., Corynebacterium parvum) have a marked stimulating effect on the reticulo-endothelial system in experimental animals. In particular, the previous injection of such suspensions will afford partial or complete protection against a later challenge with tumor cells. Up to now most research on the subject has been concerned with elucidating the course of the cellular response in animals, produced by these bacterial suspensions, and little or no systematic investigations have been made into the nature of the substance responsible, or the conditions for its production. However, it would be of considerable potential significance in cancer therapy, and also help to clarify the mechanisms of phagocytosis, if it could be purified and identified. We propose, therefore, to investigate the nature and conditions of production of this substance, its location in the bacterial cell, and methods for extracting and purifying it.